docs: def compile_struct(fmt, cache=None):
      """
      returns a struct.Struct instance compiled from fmt. If fmt has
      already been compiled, it will return the previously compiled
      Struct instance from the cache.
      """
  
      if cache is None:
          cache = _struct_cache
  
      sfmt = cache.get(fmt, None)
      if not sfmt:
          sfmt = Struct(fmt)
          cache[fmt] = sfmt
      return sfmt